Population-specific reference equations for fractional exhaled nitric oxide in healthy jordanian adults: a cross-sectional study using GAMLSS

Fractional exhaled nitric oxide (FeNO) is a biomarker of type 2 airway inflammation used in asthma diagnosis and management. Clinical guidelines use fixed decision thresholds, whereas FeNO reference distributions vary with demographic and physiological characteristics. This study aimed to develop sex-specific reference equations and upper limits of normal (ULN) for healthy Jordanian adults using the Generalized Additive Models for Location, Scale, and Shape (GAMLSS) framework. In a cross-sectional study, 906 healthy Jordanian adults (505 males, 401 females) were recruited from hospitals, pharmacies, and a university. Medical history, including asthma and atopy, was self-reported. Exclusions included current or former smoking, relevant respiratory disease, and corticosteroid or antihistamine use. FeNO was measured using a NObreath® analyzer per ATS/ERS standards. Reference equations were derived with GAMLSS. Sex-specific 95th-percentile quantile regressions provided calculator-ready formulas. Median FeNO was higher in males (17.6 ppb, IQR 13.7–22.2; p < 0.001) than females (12.0 ppb, IQR 9.0–17.0); the overall median was 15.0 ppb (IQR 11.4–20.6). The spline-based BCPEo model achieved likelihood-based pseudo-R 2 values of 0.167 (males) and 0.106 (females), outperforming the corresponding linear models. The proportion above the model-based ULN₉₅ was 5.9% in males and 4.7% in females. FeNO ≥50 ppb was rare (0.44% overall). Sex-specific FeNO reference models and upper limits of normal were derived for the sampled healthy Jordanian adults. These estimates provide a basis for evaluating local FeNO interpretation, but their external validity and diagnostic utility require assessment in independent healthy and clinical cohorts.